The answer is: there is no difference. The price for one 16-ounce bottle is $3.36. Therefore, the price for 1 ounce will be: 16 ounces : $3.36 = 1 ounce : x. x = $3.36 * 1 ounce : 16 ounces. x = $0.21. So, in this case 1 ounce is $0.21. The price for two 10-ounce bottles with the coupon for 40 cents off is: 2 * $2.30 - $0.40 = $4.60 - $0.40 = $4.20. Therefore, the price for 1 ounce will be: 20 ounces : $4.20 = 1 ounce : x. x = $4.20 * 1 ounce : 20 ounces. x = $0.21. So, in this case 1 ounce is $0.21. Thus, there is no difference if he buys one 16-ounce bottle or when he buys two 10-ounce bottles.
